The therapy of chemically induced solid tumors, through the stimulation of immunologic mechanisms in the mouse model, is the basis for this work. It is proposed to inquire whether specific immunotherapy regimens using the thymic hormone, thymosin, will accelerate the maturation rate of the thymus-derived lymphocytes and bring about an increased flux of antitumor effector cells into the tumor site. The increased generation of cell-mediated tumor rejection cytotoxicity will be monitored by quantitating the changing content of tumor macrophages and the eventual appearance of delayed-type hypersensitivity to the tumor antigens.